The present invention relates to a paste for preparing fired-on layers, in particular in the microelectronics industry, containing finely divided metallic and/or non-metallic powders and a binder of a water-soluble organic polymer and a water-soluble organic solvent.
Switching circuits for microelectronics and also for rear window heaters for cars are generally produced in a mass production process using these types of paste. For this purpose, the pastes are applied to the corresponding substrate and dried at temperatures of up to 200.degree. C. in order to remove volatile constituents. During a subsequent firing process at temperatures up to 800.degree. C., all of the other organic substances are removed and the layer is thereby compacted.
Pastes for producing thick fired-on layers using a silk screen process generally contain a large proportion of organic solvents and polymeric binders. Some of the solvents used present a health risk. Special occupational safety measures have to be adopted, therefore, when processing the pastes and cleaning the equipment which has been used. In addition, considerable amounts of solvent and organic pyrolysis products are released when drying and firing the layers and these may also be harmful to the health of workers.
A screen printable paste of this type is described, for example, in U.S. Pat. No. 5,242,623. It consists of finely divided, electrically conductive metal powders in an organic binder which is composed of several organic polymers such as polyacrylates, polyesters, polyamides, polyvinyl alcohol or ethyl cellulose, in an organic solvent such as high-boiling alcohols or terpenes.
Recently, efforts have been made to replace at least some of the solvent with water in order to minimize the amounts of health hazardous substances and in order to enable the removal of paste residues with water.
U.S. Pat. No. 5,492,653 describes a paste for the application of electrically conductive layers onto substrates in the electronics field, these consisting of silver platelets, a water-soluble polymer, in particular a polyacrylic compound, water and a water-soluble organic solvent, such as aliphatic alcohols with 6 to 18 carbon atoms, which has a boiling point higher than that of water. This paste, however, is not suitable for screen printing and produces health hazardous pyrolysis products during the firing process.
The spray process used in U.S. Pat. No. 5,492,653 is greatly inferior to the screen printing process with regard to product quality. This applies in particular to contour sharpness and surface roughness. Stringent specifications for these types of products can only be satisfied by using the screen printing process.
A water-based screen printing paste is also described in WO 9603466. The rheology of the pastes is adjusted for screen printing applications by using alkali-swellable polymeric binders. Due to the high volatility of the main solvent, water, however, the processing time is very restricted. The paste dries out and continuous use in a mass production process is thus impossible.
Therefore, screen printing pastes with organic solvents which are not very volatile are still being used.
Water is also unsuitable as the main solvent in other applications where retention of the rheological properties over a long processing time is required, despite working under normal atmospheric conditions, such as for example for the edge-metallization of capacitors.
Although a paste in accordance with EP-OS 0 630 031 can be removed with water, it contains, as solvent, organic liquids whose rates of evaporation are slower than that of water by a factor of at least 2. It therefore preferably contains higher glycols as solvent and polyvinylpyrrolidones and polysaccharides as binders. These pastes have the disadvantage that, when they are stored for progressively longer times, they form a solid sediment which can no longer be stirred up.
Thus, an object of the present invention was to provide a paste for producing fired-on layers, in particular for the microelectronics industry which avoids problems encountered in the past.
A further object of the present invention is to provide a paste which does not form a deposit over relatively long storage times, or forms a deposit which can be stirred up, and which forms the fewest possible health hazardous decomposition products.